1. How do ultrasonic through beam sensors work?
Ultrasonic through beam proximity sensors (also known as proximity switches) are used to detect the presence or absence of a target object. They consist of an ultrasound emitter and detector. The emitter is excited with high frequency AC electrical current, causing it to emit high frequency ultrasound, which is detected by the detector. When a target enters the space between the emitter and detector, blocking the ultrasound, a reduction in sound intensity is recorded by the detector. If a preset threshold intensity is crossed, the sensors output will switch states to indicate the presence of the target. Ultrasonic through beam Proximity sensors are not capable of measuring distance but rather only the presence or absence of a target. Due to the need to align the emitter and detector, they are more difficult to setup then many other proximity sensors and switches.
The emitter and detector are normally piezoelectric transducers, which are crystals that either vibrate in response to an AC voltage (thereby emitting ultrasound) or emit a voltage in response to a vibration (thereby detecting ultrasound). The emitted ultrasound normally has a frequency of 100-400 kHz. The wavelength of the ultrasound places a practical limit on the sample rate and so sensors operating at high ultrasound frequencies provide high sampling rate. However, high frequency sensors also have shorter measurement ranges because the attenuation of ultrasound is much greater at high frequencies.
2. General characteristics of ultrasonic through beam sensors
Ultrasonic through beam proximity sensors are low cost, highly reliable sensors capable of detecting the presence of targets (but not measuring their distance) at distances of 0 to 15 m, though most are limited to 2 m or less. They provide a Boolean switching output at a low sampling rate of 100-500 Hz, limiting the maximum target velocity. Ultrasonic through beam proximity sensors are non-contact and solid state meaning that they do not experience mechanical wear and have an extremely long lifetime. Furthermore, they have excellent environmental protection (IP67) and their readings are largely unaffected by contamination of the target with dirt, debris or liquid. The emitter and detector must each be separately mounted and aligned. However, fork configuration sensors are available for short sensing ranges, in which the emitter and detector are permanently fixed to a common frame.
3. Input and output signals
Ultrasonic through beam proximity sensors operate on a 10-30 Vdc supply voltage. With the exception of fork type sensors, the emitter and detector are separately wired to the voltage supply. The emitter may have additional inputs such as a beam enable/disable. The emitted ultrasonic beam is detected by the detector which outputs a Boolean signal of 0 V for a logic 0 or the supply voltage (less the voltage drop across the transistor) for a logic 1, in accordance to whether the threshold sound intensity has been crossed. The output may be either normally closed or normally open, with some sensors having both normally closed and normally open outputs.
4. Applications of ultrasonic through beam sensors
Ultrasonic through beam proximity sensors are frequently used for counting parts on production lines. Laser through beam sensors are more popular than ultrasonic through beam sensors because of their higher reliability and ability to detect smaller objects. However, ultrasonic through beam sensors are useful for detecting transparent parts and for operation in environments with either significant background light or in which laser light is unwanted. Ultrasonic through beam proximity sensors are often used to detect if multiple sheets have been inadvertently fed to a printer because if their sensitivity is adjustable, they are able to differentiate between single and multiple sheets.
5. Typical specification
|Cost||Low to medium cost|
|Measurement range||0-2 m (up to 15 m available)|
|Sample rate||10– 500 Hz|
|Acoustic frequency||40-400 kHz|
|Ambient temperature||-25 to 70 °C|
|Supply voltage||10-30 Vdc|
|Output voltage||Boolean NO/NC|
|Passive / active||Active|
|Contact / non-contact||Non-contact|
6. Purchasing tips
- Range vs Resolution: Operating at a low ultrasound frequency improves detection range but reduces sampling rate. Conversely, a high ultrasound frequency improves sampling rate but reduces detection range. There is therefore an inherent trade off between range and sampling rate.
- Adjustable threshold: Many proximity sensors include an adjustable threshold. This is useful for sensing objects that are small or thin and therefore block less of the ultrasound.
- Narrow beam: Ultrasonic proximity sensors are available with openings as small as 3 mm in diameter, for use in confined spaces.
- Pilot light: It is possible to purchase an ultrasonic through beam proximity sensor with a visible pilot light on the emitter, which projects onto the detector, aiding in alignment by providing visual feedback during installation.
- PNP/NPN: The transistor used to switch output states can be of either PNP or NPN type (also known as current sourcing and current sinking types respectively). When interfacing the sensor to a PLC, it is vital that the sensor is of the opposite type to the PLC port e.g. a current sourcing sensor will only work with a current sinking PLC port.
7. Advantages of ultrasonic through beam sensors
Ultrasonic through beam proximity sensors:
- Are low cost and highly reliable proximity sensors/switches.
- Are insensitive to background light and fairly immune to electromagnetic interference.
- Have high a high degree of environmental resistance, with a typical ingress protection rating of IP67.
8. Disadvantages of ultrasonic through beam sensors
Ultrasonic through beam proximity sensors:
- Must be installed at two points of the system (emitter and detector), both of which require wiring.
- Can be difficult to install (with the exception of fork type proximity sensors) because of the need for alignment of the emitter and detector.
- Have a smaller measurement range than laser thorough beam proximity sensors.
- Have a relatively low sampling rate due to time taken for the ultrasound to reach the emitter. This limits the maximum target velocity.
9. Application tips
- Double sheet detection: Ultrasonic through beam proximity sensors are often used in the printing industry to detect if multiple sheets of papers have been inadvertently fed to the printer. This is possible because a sheet of paper only partially blocks the ultrasound. The switching threshold can therefore be adjusted to switch only if two or more sheets are present.
- Multiple sensors: Multiple proximity sensors can be connected in series or parallel to a single process controller input port to achieve logic functions. Two sensors connected in series act as an AND gate. Two sensors connected in parallel act as an OR gate. The circuits used to implement parallel and series connections can be found here.
- Side lobe: The ultrasonic wave diverges as it propagates, becoming gradually wider. However, there is a wide angle ultrasonic emission called a side lobe, close to the sensor face. Reflection of the side lobe by surrounding objects may increase the detected sound intensity.